High vacuum device



May 10, 1960 G. T. BLACKBURN 2,936,107

HIGH VACUUM DEVICE Filed June 14, 1956 nEam 0:30am

L000 COT-.05 UK ATTORNEY men VACUUM nnvrcn George T. Blackburn, Reading,Mass.,-assignor to Na tional Research Corporation, Cambridge, Mass., acorporation of Massachusetts Application June 14, 1956 Serial No.591,317 2 Claims. (Cl. 230-45) This invention relates to a high vacuumdevice and, in particular, to a vacuum pumping system. In recent years,the use of blowers (such as Roots blowers) in vacuum pumping systems hasbecome desirable as the size of vacuum processing systems has increased.Such blowers have high volumetric efiiciency and can handle enormousquantities of gases at relatively low pressures. Such blowers areusually backed up by a conventional mechanical vacuum pump ofconsiderably smaller volumetric pumping capacity.

In the past, fairly complex arrangements have been provided forby-passing the blower during initial pump down of the system. This hasbeen due to the fact that the theoretical pumping capacity of the bloweris so much greater than that of the backing pump (or secondstage pump,as it is sometimes called) that a tremendous amount of power is requiredto drive the blower at high pressure. A lot of this power is expended inheating the gases being pumped, often with disastrous overheating of theblower. Patent No. 2,492,075 is illustrative of one system whichprovides separate drives for the vacuum pump and the blower with aby-pass around the blower through which the system is evacuated duringthe initial pump down.

It is a principal object of the present invention to provide a system ofthe general type described above which is relatively inexpensive, has ahigh capacity and is simple to operate and maintain. In particular, thepresent invention provides for the elimination of considerable piping,valves and the possibility of leaks therein.

Other objects of the invention will in part be obvious and will in partappear hereinafter.

The invention accordingly comprises the apparatus possessing theconstruction, combination of elements and arrangement of parts which areexemplified in the following detailed description and the scope of theapplication of which will be indicated in the claims.

For a fuller understanding of the nature and objects of the invention,reference should be had to the following detailed description inconjunction with the accompanying drawing, which is a diagrammatic,schematic illustration of one preferred embodiment of the invention.

In the present invention, the vacuum pumping system includes a blower ofthe Roots type or of the axial flow type as illustrated in US. Patents2,287,716 and 2,345,780, and a mechanical backing pump whose inlet isdirectly connected to the outlet of the blower. The blower preferablyhas a pumping capacity on the order of ten times the pumping capacity ofthe backing pump. Each pump has its own electric motor, the blower motorbeing of a size suflicient to drive the blower at maximum volumetriccapacity only when the blower is operating as a vacuum pump (i.e., inthe pressure range below about 15 mm. Hg abs. as measured at the bloweroutlet). In the present invention, the main blower motor is not used todrive the blower until this low pressure has been reached in the pumpingsystem. However, since the blower constitutes an impedance in thepumping system nited States Patent-O 2,936,107 Patented May 10, 1960between the inlet of the backing pump and the system to be evacuated, itis desirable to drive the blower at a relatively slow speed, as, forexample, one tenth of its normal speed, while the system is being pumpeddown to 15 mm. Hg abs. or thereabouts. This is accomplished by providinga third motor, which is preferably a fractional horsepower geared motor.The reduction gear of the fractional horsepower motor is preferablycoupled to the drive shaft of the blower through a clutch which can bedisengaged to permit the main blower motor to drive the blower at a highspeed when the required low pressure has been obtained. In a preferredembodiment of the invention, the clutch is an overriding clutch, whichpermits the main blower motor to start while the small motor is drivingthe blower and also permits the main blower motor to rapidly take overthe drive of the blower and get the blower up to speed irrespective ofthe fact that the small fractional horsepower motor is still operating.

Referring now to the drawing there is illustrated one specific preferredembodiment of the invention wherein the blower is indicated at 10 ashaving an inlet 12 which is adapted to be connected to a system 14 to beevacuated. The blower has an outlet 16 which is directly coupled to theinlet 18 of the backing pump 20. The backing pump is arranged todischarge air from the system directly to the atmosphere through outlet22. The backing pump 20 is driven by an electric motor 24, while theblower 10 is driven by the main blower motor 26 connected to blowerdrive shaft 28. The drive shaft 28 is also connected to an auxiliarydrive shaft 30 by means of a belt 32, the auxiliaiy shaft 30 beingconnected through a clutch 34 to a second shaft 35 connected to areduction gear 36, which is in turn driven by a fractional horsepowermotor 38. The motor 38 and gear 36 are preferably an integral unit. Theclutch, 34 is preferably an overriding clutch which is disengaged whenthe shaft 30 rotates faster than the shaft 35. In one embodiment of theinvention, the operation of blower motor 26 is controlled by apressure-responsive switch 40, this switch energizing motor 26 when apredetermined low pressure (e.g., 15 mm. Hg abs.) has been reached atthe inlet .to blower 10. Manual switches 42 are employed for controllingoperation of motors 24 and 38 from the main power lines which areindicated at 44.

In one specific embodiment of the invention, the blower 10 has acapacity of 1000 c.f.m. and is arranged to be driven at a speed of 3000r.p.m. with an inlet pressure range of 1 mm. Hg abs. to about 0.01 mm.Hg abs. and a compression ratio of about 10 to l. The backing pump has acapacity of c.f.m. at a speed of 335 r.p.m. with an inlet pressure rangeof about atmosphere to 10 mm. Hg abs., the outlet being to atmosphere.With this arrangement, the main blower motor 26 is a 10 HR 3600 r.p.m.electric motor directly coupled to the blower. Motor 24 is a 5 HP. 1800r.p.m. motor directly coupled to the backing pump. Motor 38 is a /2 HP.1800 r.p.m. motor and reduction gear 36 has a ratio of about 9.5 to 1,so that blower 10 is operated at about r.p.m. by motor 38. With thisarrangement, the backing pump motor 24 and fractional horsepower motor38 are started together by operating the two switches 42. This causespump 20 to operate at 1800 r.p.m. and blower 10 to operate at 190 r.p.m.As the pressure in inlet 12 is reduced (due to the evacuation of system14) to a pressure on the order of 15 mm. Hg abs., the pressure switch 40is operated and main blower motor 26 is started, this motor rapidlybringing shaft 28 and blower 10 up to operating speed of 3000 r.p.m. Assoon as shaft 28 increases its speed, clutch 34 is disengaged and motor38 can be stopped.

Since certain changes may be made in the above companying drawing, shallbe interpreted as illustrative,

and not in alimiting sense.

What is claimed is:

1. In a vacuum pumping system of the 'type'which includes a high vacuumblower and a mechanicalbacking pump connected to the outlet of theblower,the inlet to the blower being arranged to'be connected to asystem to be evacuated and the outlet of the backing pump being arrangedto discharge to atmosphere and wherein separate motors are provided fordriving the blower and backing pump, the improvement which comprises athird motor for driving said blower, 'saidthird'motor'havingsubstantially less power thanthe blower motor, a reduction gear betweenthe 'thirdmotor'anda driveshaft for the blower so that thethirdmotor'can drive the blower at a speed substantially lessthan'thespeed-at 'whichth'e blower has optimum vacuum pumping "action,and a clutch between the reduction'gear and the 'blower drive shaftwhich can be disengaged to permit-the'blower'motor to drive the blowerat a high speed when the blower 'is disconnected fromthe reductiongear.

"2. In a vacuum pumpingsystem of the type which includes a highvacuum'blower and a mechanical backing pump connected to the outlet ofthe blower, theinlet to the blower being arranged to be connected to asystem 'to-be evacuated and-the outlet of'the-backing pump beingarrangedto discharge to atmosphere and wherein separate motors are provided fordriving the blower and backing pump, the improvement which comprises athird motor for driving said blower, said third motor being a fractionalhorsepower motor, a reduction gear between the third motor and a driveshaft for the blower so that the third motor can drive the blower at aspeed on the order of one-tenth and less of the speed at which theblower has optimum vacuum pumping action, and an overriding clutchbetween the reduction gear and the blowerrdrive shaft which can bedisengaged to permit the blower motor to drive'theblower at a high speedwhen the blower is disconnected fromthe reduction gear.

References Cited in the file of this patent UNITED STATES PATENTS1,376,411 Eaton May 3, 1921 2,356,590 -'Jacobsen Aug. 22, 1944 2393;010Arnold Jan. 15, 1946 2492,07 5 Van Atta Dec. 20, 1949 246763196 *GilbertApr. 27, 1954 FOREIGN PATENTS 1,126,189 'France July 23, 1956

